This application claims the priority of German Application No. 199 42 815.8, filed Sep. 8, 1999 and International Application No. PCT/EP00/08244, filed Aug. 24, 2000, the disclosures of which are expressly incorporated by reference herein.
The invention relates to a device for actuating an airbag having a controller, a firing device for firing the airbag and electric leads which connect the controller to the firing device, a coil for damping high-frequency interference currents being connected into the electric leads, which coil can in turn induce interference currents in the leads to the firing means if an external electromagnetic interference field is present.
For actuating airbags in motor vehicles, circuits are known which have a controller via which a firing device for firing the airbag is actuated. A firing device and be firing caps which fire, for example, a gas cartridge if the actuation current at the input of the firing cap exceeds a threshold value.
In order to attenuate the interference currents which were induced in the electric leads by strong electromagnetic fields, a coil is already connected into the electric leads between the controller and the firing cap. The coil reduces, in particular, high-frequency interference currents in the leads. The coil is electrically connected to the firing cap in such a way that the length of the electric leads is at a minimum. In the case of an external magnetic field, not generated by the coil itself, the coil induces interference currents in the electric leads. The behaviour is comparable to the secondary winding of a transformer if the magnetic flow flowing through the core is considered as the external electromagnetic field.
German Patent DE 299 00 178 U1 discloses a seat-belt pretensioning device with a gas generator, in which a circuit with a coil is provided within a cylindrical housing, as a result of which relatively high frequency interference signals cannot get to the ignition system.
German Patent DE 4 136 248 A1 discloses a coil plug which has two coils wound in opposite directions around the same core within a plug housing. The coils are arranged in electric leads which are independent of one another, neither the wiring nor the function of the two coils being explained in more detail.
The object of the present invention is to configure and develop a device of the type mentioned above in such a way that the external effects of a magnetic field on the coil are compensated and, as a result, the interference currents in the leads are reduced.
Accordingly, in order to compensate the interference current caused by the external magnetic field in the coil, a compensation coil is connected into the electric leads and arranged in the vicinity of the coil in such a way that the induced compensation currents caused as a result of the external magnetic field in the compensation coil are opposed to the interference currents. As a result, the currents caused by the external magnetic field cancel one another out and in addition, as in the prior art also, the high-frequency currents within the electric leads are filtered out.
According to the invention it has been recognized that the influence of an external magnetic field in the coil can be advantageously compensated by a compensation coil. Here, in addition, by virtue of suitable dimensioning, the high-frequency currents in the electric leads are reduced by the alternating current resistance of the coils connected in series, as was already the case when there was only one coil. At the same time, the coils which are arranged antiparallel considerably reduce the overall interference currents in the coil which are induced as a result of an external magnetic field. The compensation coil thus reduces the negative effects which are produced owing to the external interference field.
The coil and the compensation coil are preferably provided in the same lead and connected in series there. As a result, the external electromagnetic field in the two coils generates the currents which are the same, but act in opposite directions. By means of the dimensioning, embodiment and arrangement of the coils, it is then possible for the currents, which are induced by the external interference field, to be directed in opposite directions and have the same strengths so that they cancel one another out. Therefore, an external interference field does not result in any additional interference currents at the load to be protected, in particular a firing cap. As a result of the arrangement of the coils, the high-frequency interference currents in the leads and the currents induced by an external magnetic field in the coil are minimized.
The compensation coil is preferably arranged antiparallel with respect to the coil. Because the two coils are connected in series, the windings of the two coils then run with respect to one another in such a way that coil fields in opposite directions are produced. For this purpose, on the one hand, the height of the winding and on the other hand the sense of the winding in the winding direction can be adapted.
In order to change the induced current and thus determine the compensation behaviour of the compensation coil, the material of the coil core or the orientation of the coil winding or the number of windings can be adapted. Because the same current flows through the windings of the two coils because the series connection of the coils, the coil field is preferably adapted by means of the parameters mentioned above. For this reason, the induced total current of differently wound coils can still compensate one another (become zero) if, for example, different materials are used for the two coils and their electromagnetic fields are tuned for compensation.
In another development of the invention, the coil core is embodied as a closed core around which either the coil or the compensation coil or both coils are wound. When only a single annular core is used, the coil and the compensation coil can be combined to form a composite coil so that only the composite coil is wound around the annular core. The coil core is in particular embodied as an annular core. The annular core can have here a circular cross section or a cross section which is essentially rectangular, the annular core being cylindrical. The annular core then has the shape of a torus which is approximated to a lateral surface of a cylinder. In this case, a coil winding is not essentially circular but rather of rectangular or oval construction in accordance with the cross section.
With respect to an embodiment with two annular cores it has been found that when there is one coil wound on the first annular core and one compensation coil wound on the second annular core, the two coils are each provided only on one half of an annulus. If, given annular cores which are arranged in parallel one next to the other, they are considered in the direction of their common axis, the coil on the front annular core is, for example, wound in the viewing direction only around the right-hand side of the annulus and the compensation core on the rear annular coil is wound only around the left-hand side of the annulus.
The winding heights, i.e. the pitches of the windings, are arranged at an equal distance from one another. As a result, the induced total current as a result of an external magnetic field is effectively eliminated. For an optimum function, the pitches of the windings must be arranged at the same distance from one another, specifically over the entire casing of the core. Any deviation from this uniform winding degrades the desired compensation result.
In a particularly preferred development of the invention, a plug is provided in the leads between the controller and the firing means and a coil is arranged within the plug or in the region of the plug. The coil can be arranged within an insulating material, specifically even on the outside of the plug, but an arrangement of the coil within the housing of the plug is of particular advantage. In this case it is particularly advantageous to encapsulate the coil. For this purpose, the housing can be adapted to the outer shape of the coil. Depending on the outer shape of the coil, the housing is then essentially cylindrical or cuboid and/or for example also water-repellent.
According to the invention, the coil and the compensation coil can be arranged within the housing of the plug. It is then best to construct the housing of the plug in two parts and can be opened out when it is equipped with plug parts. Both coils can be arranged within the housing with axes which are parallel with one another. For this purpose, two closed or interrupted annular cores can be arranged adjacent to one another. The coil can be wound around an annular core and the compensation coil can be wound around the other annular core, and both coils can be insulated from one another within the housing. Because both coils are connected in series, a lead is first wound around a coil core in order to form a coil and then around the other coil core in order to form the other coil, or the compensation coil. Both coils can be wound on half a side in the viewing direction parallel to the common axis of the annular cores, and offset laterally with respect to one another. As a result, the capacitive coupling of the two coils is reduced. This is advantageous if the interference currents are coupled in the region of the feeder line to the coils and a sufficiently good damping of the interference is to be achieved.
As indicated previously, it is, however, also possible to provide just one annular core onto which a composite coil is wound, said composite coil forming the coil in one part and the compensation coil in the other part. The effects generated by an external field in the part of the composite coil which forms the compensation coil eliminate the effects formed by the same external electromagnetic field in the other part of the composite coil. The other part of the composite coil forms here the coil which was originally provided for filtering the high-frequency currents.